Fighting Flutter

“I worked on a program in the 1980s where flutter really bit us, and that program was eventually canceled,” recalled Ed Burnett, the X-56A unmanned research aircraft technical program manager for the Lockheed Martin Skunk Works. “It became a personal interest of mine ever since then to kill flutter.”

Flutter occurs at the point where aerodynamic forces acting on an aircraft in flight and the structural dynamic properties of the vehicle combine, or couple, to produce an often violent, harmonic vibration. The resulting vibration creates a potentially catastrophic condition that can quite literally break a wing or tail surface off an aircraft.

In simple terms, think of a flag on a pole. The wind blows at a specific speed, but the flag, with a specific set of structural properties, doesn’t wave uniformly from side to side. It usually dips, or swells, or bends. When conditions are right to cause the flag to pulsate, the flag is basically experiencing induced flutter. If the wind is too strong and the grommets or stitching fail, the flag is ripped from the pole.

Flutter has been a known hazard almost from the beginning of powered flight. Even with more than a century of aircraft design experience, conditions where flutter occurs can still be hard to predict precisely. Aircraft designers traditionally have dealt with flutter by trying to avoid it—through such methods as adding structure.

By building enough stiffness into a specific structure, the natural tendency of structural dynamics and aerodynamics to couple can be prevented—even at conditions way outside an aircraft’s normal operating flight envelope. But the increased stiffness also adds weight—sometimes considerable weight—which, in turn, increases fuel use. And the end result—reduced range.

The X-56 team hopes to do something that is completely different. “The aim of the X-56A program is to mature flutter suppression technologies,” notes Burnett. “If we can suppress flutter by using the same flight control technologies used to provide vehicle stability, designers can use longer, more flexible wings and lighter weight structures. That will allow future aircraft to fly higher, faster, and farther than before.”